Does this violate conservation of momentum?

[davey2222]

I copied and pasted the following from Unexplained-mysteries.com:

 

"Imagine an electromagnet, hooked up to some energy source. Attached to it by a frame is a piece of iron, at some distance from the magnet. The electromagnet is switched on for a fleeting instant, and then switched off again. Its magnetic field travels out in all directions as a wave, propagating through space at light speed, about 300,000 kilometers per second. When it reaches the piece of iron, the iron is pulled toward the electromagnet, and its own magnetic wave travels back to the electromagnet. But by the time the wave reaches it, the electromagnet has been switched off…so it is not attracted toward the iron. The forces within the system are unbalanced, and there is an “impossible” net momentum in one direction. The entire apparatus, including the piece of iron, is accelerated in that direction."

[pmb]

I copied and pasted the following from Unexplained-mysteries.com:

 

"Imagine an electromagnet, hooked up to some energy source. Attached to it by a frame is a piece of iron, at some distance from the magnet. The electromagnet is switched on for a fleeting instant, and then switched off again. Its magnetic field travels out in all directions as a wave, propagating through space at light speed, about 300,000 kilometers per second. When it reaches the piece of iron, the iron is pulled toward the electromagnet, and its own magnetic wave travels back to the electromagnet. But by the time the wave reaches it, the electromagnet has been switched off…so it is not attracted toward the iron. The forces within the system are unbalanced, and there is an “impossible” net momentum in one direction. The entire apparatus, including the piece of iron, is accelerated in that direction."

I don't see any reason to assume that the momentum is unbalanced. When the EM wave hits the piece of iron the iron radiates an EM wave in all directions, not just back to the source. The stress wave inside the frame does not move at c so it hits the magnet later. When the wave hits the magnet it will stimulate another EM wave and it goes on and on. The momentum of any system is always conserved.

 

See http://home.comcast.net/~peter.m.brown/em/momentum_of_radiation.htm

[swansont]

IIRC this is an issue with classical E&M, and there has been discussion on it in terms of advanced and retarded potentials. One issue that's apparent is that even if the electromagnet were left on, the force on it wouldn't occur until the round-trip time of t=2L/c. I suspect this is an issue that requires QED to properly address.

[Andrius319]

Hi,

Does anyone have any sources/pappers about investigation on this topic?

[swansont]

2 hours ago, Andrius319 said:

Hi,

Does anyone have any sources/pappers about investigation on this topic?

The classical basics can be found in Jackson's E&M textbook, which is a graduate-level text.

[Andrius319]

18 hours ago, swansont said:

The classical basics can be found in Jackson's E&M textbook, which is a graduate-level text.

Great, thx ^-^

Hope there this question will be discussed sufficiently

[studiot]

On 24/05/2012 at 4:39 AM, davey2222 said:

I copied and pasted the following from Unexplained-mysteries.com:

 

"Imagine an electromagnet, hooked up to some energy source. Attached to it by a frame is a piece of iron, at some distance from the magnet. The electromagnet is switched on for a fleeting instant, and then switched off again. Its magnetic field travels out in all directions as a wave, propagating through space at light speed, about 300,000 kilometers per second. When it reaches the piece of iron, the iron is pulled toward the electromagnet, and its own magnetic wave travels back to the electromagnet. But by the time the wave reaches it, the electromagnet has been switched off…so it is not attracted toward the iron. The forces within the system are unbalanced, and there is an “impossible” net momentum in one direction. The entire apparatus, including the piece of iron, is accelerated in that direction."

 

Well the original authors can prove anything they like by presenting either insufficient or invalid information or both.

 

For starters the magnetic field is not a wave it is a (single) pulse.

This will be subject to all the standard pulse characteristics such as rise time and decay time, which will be determined by the characteristics of the electromagnet.

The iron is massive so will take time to "be pulled towards the electromagnet", yet the time intervals of transit by the pulse edges must be very small.

[swansont]

1 hour ago, studiot said:

 

Well the original authors can prove anything they like by presenting either insufficient or invalid information or both.

 

For starters the magnetic field is not a wave it is a (single) pulse.

This will be subject to all the standard pulse characteristics such as rise time and decay time, which will be determined by the characteristics of the electromagnet.

The iron is massive so will take time to "be pulled towards the electromagnet", yet the time intervals of transit by the pulse edges must be very small.

I don't think this is relevant.  The time it takes for the magnetic field to appear at the iron is L/c, so if L is 1 light-second, then there is a 1-second delay. The OP's scenario has the electromagnet being turned off before 1 second.

But we're talking about the EM interaction, which means virtual photon exchange (QED), and I suspect that the fact that the magnet was turned off doesn't matter regarding conservation of momentum. But I don't know enough QED to walk through it.

[studiot]

18 minutes ago, swansont said:

I don't think this is relevant.  The time it takes for the magnetic field to appear at the iron is L/c, so if L is 1 light-second, then there is a 1-second delay. The OP's scenario has the electromagnet being turned off before 1 second.

But we're talking about the EM interaction, which means virtual photon exchange (QED), and I suspect that the fact that the magnet was turned off doesn't matter regarding conservation of momentum. But I don't know enough QED to walk through it.

On the contrary, I think it to be very relevant.

For instance: By how much will the mag field (pulse) have weakened or diminished over a distance of 300,000 kilometers?

[swansont]

1 hour ago, studiot said:

On the contrary, I think it to be very relevant.

For instance: By how much will the mag field (pulse) have weakened or diminished over a distance of 300,000 kilometers?

It doesn't matter. The magnitude of the signal isn't the issue.

[studiot]

52 minutes ago, swansont said:

It doesn't matter. The magnitude of the signal isn't the issue.

Of course it matters, but it's only part of the story.

 

Here's another part.

 

 

One part of a (perhaps rigid but that's yet another part) body of ridiculously large dimensions labelled B emits momentum carrying particles.
There must be a commensurate reaction with the frame at that point.

 

After a time period some - a ridiculously small % - of this momentum is transferred back to the body at A.
There must be a commensurate reaction with the frame here.

The fronts of both reaction forces must travel through the frame much more slowly than c, but their effect cannot be discounted.
This is always the case with bodies of sufficient size - one part of the body reacts before another.
Ther were actually some tremendous slomo videos of springs suspended from building and dropped. The time taken for the effect of release could clearly be seen in the responses of the top and bottom of the spring. These were posted here not too long ago, but like all the best stuff I can't find it again, it seems hidden from my searches.

I view the authors (note not the OP here) as trying to promote yet another perpetual motion machine.

 

Edited April 10, 2018 by studiot

[swansont]

1 hour ago, studiot said:

Of course it matters, but it's only part of the story.

 

Here's another part.

 

 

One part of a (perhaps rigid but that's yet another part) body of ridiculously large dimensions labelled B emits momentum carrying particles.
There must be a commensurate reaction with the frame at that point.

 

After a time period some - a ridiculously small % - of this momentum is transferred back to the body at A.
There must be a commensurate reaction with the frame here.

The fronts of both reaction forces must travel through the frame much more slowly than c, but their effect cannot be discounted.
This is always the case with bodies of sufficient size - one part of the body reacts before another.
Ther were actually some tremendous slomo videos of springs suspended from building and dropped. The time taken for the effect of release could clearly be seen in the responses of the top and bottom of the spring. These were posted here not too long ago, but like all the best stuff I can't find it again, it seems hidden from my searches.

I view the authors (note not the OP here) as trying to promote yet another perpetual motion machine.

 

But the frame is a medium and right next to the source, so you have changed the conditions of the OP's problem. It's not a valid answer.

 

[studiot]

31 minutes ago, swansont said:

But the frame is a medium and right next to the source, so you have changed the conditions of the OP's problem. It's not a valid answer.

 

 

 

Of course I haven't.

 

On 24/05/2012 at 4:39 AM, davey2222 said:

Imagine an electromagnet, hooked up to some energy source. Attached to it by a frame is a piece of iron,

 

Which is exactly what I have drawn.

[swansont]

28 minutes ago, studiot said:

 

 

Of course I haven't.

 

 

Which is exactly what I have drawn.

You have the wave reacting with the frame, which is not part of the OP. In the OP, the concern is the wave traveling through space.

IOW the frame is incidental in the original scenario. In yours it is a critical element.

[studiot]

8 minutes ago, swansont said:

You have the wave reacting with the frame, which is not part of the OP. In the OP, the concern is the wave traveling through space.

IOW the frame is incidental in the original scenario. In yours it is a critical element.

 

The OP introduced the frame and cannot discount its effects.

That would be like me saying I know Henry's nose is bleeding Miss, but that doesn't count because although it was my fist, his nose was not part of the travelling momentum and so doesn't count.

 

In any event, no I do not have a wave reacting with the frame, it reacts with the iron and the electromagnet, which is attached (by the OP) to the frame.

[swansont]

2 hours ago, studiot said:

 

The OP introduced the frame and cannot discount its effects.

That would be like me saying I know Henry's nose is bleeding Miss, but that doesn't count because although it was my fist, his nose was not part of the travelling momentum and so doesn't count.

 

In any event, no I do not have a wave reacting with the frame, it reacts with the iron and the electromagnet, which is attached (by the OP) to the frame.

The magnetic field pulse emitted is symmetric, so there is no momentum change associated with that and thus no "reaction with the frame"

The conundrum is still in place even if there is no frame connecting the two objects. The purpose of the frame is to make this into a perpetual motion machine, as you note. But it's a distraction from the actual problem.

You could restate the problem with both the magnet and the iron hunk in free space, and of equal mass. They should meet in the middle, having been imparted an equal amount of momentum. But if the second interaction is missing, as claimed in the OP, only the iron moves. You'd still be violating conservation of momentum.

 

[Andrius319]

Swansont is right.

I have made a research beforehand (using classical electromagnetism). The results violated conservation of momentum. It would lead to incredible aplications if this is a true in practice, this is the reason I looked for other information sources for confirmation or dennial.

On other note, its quite hard to make a good experiment with it producing enough force to measure it easly.

 

If you have a link to OP's article would be interested in checking that as well.

Edited April 11, 2018 by Andrius319
Grammatical mistake in spelling name